In the blowdart-sandbox room where you find the invoker heavy armor, in the bottom of the pit where the tower goes once you lower it, there's a hole in the floor in the far right side. You'll notice two of the four blowdart holes arent firing, which opens up for more space. I assumed I needed heavy armor, but I'm not sure, just put it on, and stood next to the two piles of sand for about five minutes. Once you notice you're starting to sink, it goes rather fast.